This application makes reference to and claims all benefits accruing under 35 U.S.C. Section 119 from an application for FABRICATION METHOD OF GRADED INDEX SILICA GLASS filed earlier in the Korean Industrial Property Office on Sep. 15, 2000 and there duly assigned Serial No. 52345/2000.
1. Field of the Invention
The present invention relates generally to a silica glass, and more particularly to a fabrication method of silica glass by a sol-gel process.
2. Description of the Related Art
Optical fibers are made to exacting dimension and compositional requirements. Normally, fiber manufacturing starts with a preform, which is a large cylinder of glass with the same composition as the final fiber, but at a much larger diameter. Different methods are used to make the preform. One of the methods includes Modified Chemical Vapor Deposition (MCVD), where high-performance single-mode fiber is made. The conventional optical transferring media such as lens or optical fiber with a convex graded index are fabricated through the MCVD process. The mechanisms of the MCVD process is disclosed on pages 138-147 of J. Colloid Interface Sci. 69, which is a thesis authored by Walker, K. L., Homsy, G. M. and Geyling, F. T. in the U.S. in 1979. This publication discloses a process of fabricating a silica glass by supplying and depositing raw material gas inside of a depositing tube, to which heat treatment is applied from outside.
In particular, the heat treatment using a burner which is supplied from outside forms a temperature field inside the depositing tube. If a critical temperature is formed inside the depositing tube through the heating via the burner, a rapid oxidization reaction occurs, thereby generating glass particles. The generated glass particles form a particle orbit due to the thermophoretic force generated by the temperature field. The glass particles move toward inside the depositing tube as the temperature of inner wall of the depositing tube is higher than the temperature of the gas. Therefore, some of the glass particles are evaporated on the inner surface of the depositing tube.
Gradient-index glass is often made in the form of solid cylindrical bodies, where the index of such bodies changes radially, usually with the highest index being along the axis of the cylinder and the lowest index located at the outer periphery. When fabricating such a silica glass using the MCDV method described above, it is sometimes difficult to properly control the amount of raw material gas supplied to the depositing tube in the course of deposition process to form the gradient-index profile inside of the silica glass. In addition, it requires a very long fabrication time during the deposition, and the cost of manufacturing is high.
It is, therefore, an object of the present invention to provide a method of fabricating gradient-index silica glass with a simpler fabricating process and lower fabricating cost than the modified chemical vapor-phase deposition (MCVD) process known in the prior art system.
Accordingly, there is provided a fabrication method of graded index silica glass, comprising the steps of: dispersing by mixing a starting material with a dispersion medium to form sol; molding by putting the sol within a molding frame to form moisturized gel and separating the moisturized gel from the molding frame; drying the moisturized gel under pre-set temperature and humidity and removing the dispersion medium inside of the moisturized gel to form a first dry gel having a pre-set moisture containing dispersion; hydrolyzing to induce hydrolysis between the remaining moisture inside of the first dry gel and an additive solution by putting the first dried gel into the additive solution; re-drying to form a second dry gel by re-drying the first dry gel, which underwent the hydrolysis; and, heating by supplying a reaction gas to the second dry gel, removing impure materials therefrom, and annealing the resultant material for vitrification.